* input/regression/optimal-page-breaking-hstretch.ly: test for
ragged-last-bottom also
* lily/paper-column-engraver.cc (finalize): make the end of a score
breakable by default. This is to balance out a change in behaviour
of the page-turn-breaker which no longer makes the end of a score
breakable.
* lily/paper-book.cc (pages): set the systems_ once the pages are
broken
* lily/page-turn-page-breaking.cc (calc_subproblem): use the new
Page_breaking interface.
* lily/page-breaking.cc (class Page_breaking): make the interface
more consistent and provide abstractions for dealing with
Line_divisions.
* lily/optimal-page-breaking.cc (solve): use a more straightforward
algorithm. Use the new interface to Page_breaking.
* lily/page-spacing.cc: better support for ragged-bottom and
ragged-last-bottom